The conductivity, or insulation capability, of the insulation layer of a HVDC or HVAC cable is one of the most important parameters in the quality of cable. High conductivity of the insulation anywhere in the cable creates high risk of thermal runaway and breakdown of the cable. Besides the level of conductivity, the profile of conductivity is also a very important issue. If some parts of the cable have higher conductivity than the other part, the electric field in the parts with lower conductivity will be enhanced and this may cause breakdown. Because of the reasons above, there is a very high interest in gaining information about the level and profile of conductivity of the cable insulation. Conventional routine testing of cables will not reveal possible problems related, for example to essential DC-properties e.g. conductivity of the insulation.
Currently two methods are used for measurement of conductivity. One is to make plaque samples by press from the insulation material before it is molded or pressed on the cable and measure the conductivity of the sample. In this method the uncertainties about the similarity of conductivity in the produced plaque sample and the cable is always an issue since the sample piece can't be related to any definite place of the cross section profile of the produced insulation layer. Further, samples made of raw material do not reflect the effects of the manufacturing process to the insulation layer. Therefore the results obtained by these methods are only indicative. According to another method, the conductivity of a cable piece is measured by measuring the leakage current of the cable under DC voltage; this method gives only the overall conductivity of the cable and not the conductivity profile. As this method requires measuring a long length of a cable, it is very hard and expensive to realize as a routine measurement under production.
WO 201112847 discloses a method for controlling the conductivity of the insulation. In this method pellet samples are taken from raw material and pressed into plaque samples, whereof the measurements can be performed. This method provides indication of the properties of the raw material only and the final quality of the cable insulation can't be assessed. WO 2011057927 a same type of method.
WO 03046592 discloses a method wherein a DC voltage of 15 kV is applied between a metal screen of the cable and a semiconductive layer of a cable sample. The method requires a long sample, according to the publication a sample having length of 20 m is needed.
U.S. Pat. No. 5,416,419 discloses a method for deterioration of the insulation of a power cable.